A technology for measuring a profile in a fine region using a white-light interferometer has been widely prevailed and currently utilized in various fields.
One conventional white-light interferometer is disclosed in Korean Patent No. 10-598572. In the procedure of making semiconductor and LCD, there is a process for applying a transparent thin film layer on a surface of an opaque metal layer, and at this time, some methods are proposed to measure the thickness of the transparent thin film layer or the profile of the surface of the thin film layer.
As one method for measuring the profile of the surface of the transparent thin film layer, white-light scanning interferometry has been proposed, which overcomes 2π-ambiguity the conventional phase shifting interferometry has had, so that the measurement surface having rough surface or high step can be measured with high resolution.
The fundamental measurement principle of the white-light scanning interferometry makes use of the feature of the short coherence length of white light. This is based upon the principle where an interference signal is generated only when reference light and measurement light split from a beam splitter have the almost same optical path difference as each other.
Therefore, while an object to be measured is being moved by a fine distance of a nanometer through moving means like a PZT actuator in the direction of an optical axis, it is observed that short interference signals are generated at respective measurement points within the measurement region having the same optical path difference as a reference mirror.
If the positions where the interference signals are generated are calculated from all measurement points within the measurement region, the information on the three-dimensional profile of the measurement surface is obtained, and accordingly, the profile of the surface of the thin film layer can be measured from the obtained information on the three-dimensional profile.
FIG. 1 shows a conventional measurement apparatus using a white-light interferometer. As shown, the conventional measuring apparatus includes a light source 110, a beam splitting unit 150, an interference module 120, an imaging unit 140, a transferring unit 130, and a controller 160.
The light source 110 emits white light. The light source 110 emits monochromatic light, for example, white light, and uses an about 70 W tungsten-halogen lamp. In this case, the light emitted from the light source 110 is passed through an optical fiber (not shown) in the direction of the emission.
The light emitted from the optical fiber is distributed around a pin hole of a fixing member 171. While the light passed through the pin hole is being transmitted through a convex lens 172 disposed between the fixing member 171 and the beam splitting unit 150, it is arranged to a given width.
The light transmitted through the convex lens 172 is incident onto the beam splitting unit 150. In this case, the light incident onto the beam splitting unit 150, for example, a beam splitter is reflected to about 45° with respect to the incident direction thereof and thus headed for an object 100 to be measured.
The light reflected by the beam splitting unit 150 and headed for the object 100 is incident onto the interference module 120. The light incident onto the interference module 120 is split into the directions of a reference mirror and the object 100 provided in the interference module 120 and thus emitted. Coherent light is formed by the light reflected from the reference mirror and the object 100 and emitted to the beam splitting unit 150.
The imaging unit 140 images the coherent light emitted from the interference module 120 and then passed through the beam splitting unit 150 and a convex lens 174 and applies the imaged light to the controller 160.
The controller 160 controls the transferring unit 130 according to the white-light scanning interferometry to adjust the separation distance between the transferring unit 130 and the object 100. Further, the controller 160 measures the profile of the surface of the object 100, based upon the imaged data by the imaging unit 140 corresponding to the separation distance between the transferring unit 130 and the object 100.
However, such white-light scanning interferometer has a coherent section of about 2 to 4 μm and a period of interference pattern of about 0.3 μm, and so as to measure a three-dimensional profile having uneven heights, accordingly, step transferring should be needed at substantially short distances. Further, the interference pattern has to be acquired over the whole height, and accordingly, the time for the measurement becomes extended.
Such measurement method is effective under the situations wherein the object has a relatively low difference in height and mechanical vibrations are not generated, but under the situations wherein the object has a relatively high difference in height and mechanical vibrations are generated, it is hard to obtain an appropriate measurement result.